When a horse is afflicted with HERDA, there is a lack of adhesion within the dermis (a deep layer of skin).

Photo: Ann Rashmir-Raven, DVM, MS, Dipl. ACVS, PGCVE, FHEA

Some skin conditions in horses are caused by fungus and bacteria, while others are caused by external factors such as beddings or medications. However, some dermatological issues appear in horses courtesy of genetics. At the 2011 Western Veterinary Conference, held Feb. 20-24 in Las Vegas, Nev., Ann Rashmir-Raven, DVM, MS, Dipl. ACVS, of Michigan State University, presented on a few of the most common equine skin disorders that have a genetic basis.

Rashmir-Raven explained that hereditary skin disorders are most commonly autosomal recessive traits, meaning that each of the foal's parents, which appear normal, carries one normal allele and one defective allele. When both parents pass their defective alleles on to the foal the foal ends up with a pair of defective alleles, triggering disease expression.

She stressed that in most cases these autosomal recessive genetic disorders--which remain incurable and sometimes untreatable--can be prevented with careful breeding planning. Many DNA tests are now available to help an owner determine whether a mare or stallion is a carrier of a specific genetic disorder, and Rashmir-Raven recommends obtaining test results from both the mare and stallion to ensure two carriers are not bred (which would result in a 25% chance of a foal with the pair of defective alleles being born).

Hereditary Equine Regional Dermal Asthenia

Hereditary equine regional dermal asthenia (commonly referred to as HERDA) is one of the most common hereditary skin disorders in horses, Rashmir-Raven explained, and she spends a great deal of her research time examining this problem of Quarter Horses and related breeds. The roots of HERDA can be traced back to one stallion: a Quarter Horse foaled in 1944 called Poco Bueno.

When a horse is afflicted with HERDA, there is a lack of adhesion within the dermis (a deep layer of skin). Because the layers are not held firmly together, they separate. When the horse suffers trauma to the skin, the outer layer often splits or separates from the deeper layer, or it can tear off completely.

With a few exceptions, affected animals typically appear normal at birth. Badly affected horses begin to show signs by a year of age but most aren't diagnosed until they begin training under saddle, she explained. At that point, everyday contact--such as tacking up or untacking, wearing a saddle, a rub on a fence rail, or a bite from another horse--will cause significant lacerations, hematomas, and seromas (swelling due to localized collections of serum in the tissue) in these horses that frequently result in disfiguring scars.

Clinical signs of HERDA include:

"Stretchy" skin that feels "mushy" or "doughy" to the touch;

Skin that can be easily manipulated, especially along the horse's spine where wounds most frequently occur; and

Skin that remains "tented" for a prolonged period of time when manually stretched.

The damage from HERDA is not only skin deep, however. Rashmir-Raven noted that while the skin is the most common organ to be affected by HERDA, the disorder can affect tissue throughout the body.

There is no treatment for HERDA, she explained, and most affected horses are euthanized as their delicate skin and constant wounds can be painful for the animal and present a huge problem for caretakers.

"This is getting to be catastrophic in Quarter Horses," Rashmir-Raven said, adding that the disorder has generally been noted in the cutting, reining, and pleasure disciplines. "Fourteen of the top 100 (cutting horse) sires are carriers (of HERDA) whose offspring have earned in excess of $90 million. Currently, 28% of the top winning cutting horses are carriers. Therefore, we will soon double the number of carriers being bred when these top horses retire to the (breeding) shed."

Junctional Epidermolysis Bullosa

Junctional epidermolysis bullosa (JEB) is a fatal autosomal recessive disorder that affects mainly Belgian draft foals and some Warmblood foals. Rashmir-Raven explained that the disease causes affected foals to develop severe skin lesions over pressure points of the body (areas where the skin is stretched such as the hock, stifle, and elbow), with severely affected foals suffering complete skin loss in some areas of the body, such as the lower legs, she added.

She added that affected foals commonly have dental problems, as well, including:

Premature eruption of teeth (with the central incisors and premolars sometimes being present at birth);

Irregularly serrated edges on incisors and cheek teeth; and

Enamel pits on incisors and cheek teeth.

Rashmir-Raven noted that affected foals must be euthanized shortly after birth: "There is no treatment, so it's time to put that poor foal out of its misery."

In 2001, researchers linked the condition in Belgians to a mutation of the LAMC2 gene, which produces part of the laminin-5 protein. Laminin-5 is important in the attachment of the epidermis to the dermis as well as mucous membranes throughout the body. Shortly after the identification of the gene, a commercial polymerase chain reaction (PCR) test was developed to test DNA from the horse's main or tail hair to detect if a horse is a carrier.

Upon analysis of 252 samples from male and female Belgains, approximately 17% of the horses were found to be carriers. Currently, the carrier status of the horse is recorded on the registration papers, a feature that allows breeders to avoid breeding carriers and possibly producing a foal with JEB.

Overo Lethal White Syndrome

The subject of continued discussion in the Paint horse community is overo lethal white syndrome (OLWS), she explained. Foals with OLWS have a non-functional digestive tract and cannot pass manure. The disease is seen in all-white or primarily white Paint foals. Horses carrying the OLWS gene usually have a frame overo coat color pattern (characterized by white coloration of the abdomen that does not cross the dorsal midline between the withers and tail), although occasionally carriers are solid-colored Paint broodstock or horses with other coats colors.

Rashmir-Raven noted that there is no way to treat these foals, even surgically, so they die shortly after birth or must be euthanized. Since this is another autosomal recessive trait, she recommends genetic testing of the mare and stallion (which can be carried out at laboratories across the nation) to avoid breeding OLWS foals in the first place.

About the Author

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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